Make up air ducting question

I am running across a lot of new homes that have the fresh air make up vent terminating in the attic space. While at first glance I would agree that it is bring fresh air into the HVAC system. The question is. Is this correctly installed?
Seems to me to be a real bad idea to bring 120+ degree fresh air into the house during colling season as the AC needs to now cool all that hot air.
I cannot seem to find an answer to this question so I though I would post it here and see what all you experts think.


Hi John,
A well ventilated attic can be used as a source of combustion air. Check 2003 IRC section R806 for more detail. I don’t have the book with me, I just know it’s in there.

Erol Kartal

i’d question the lack of filter. plus any little tree rat could just jump in and cause all kinds of havok.

Would it be any different from the furnace sitting in the utility closet with a screened ceiling to the attic?

Yes, I would put a screen over it, though.

When combustion air is taken from an attic the duct must extend a min. of 6 inches into the attic above the ceiling joist and insulation and the attic must be well ventilated. NO SCREENS to be used on the duct ends. IRC M 1703.3

It’s a good thing we don’t use the IRC here. I’m putting a screen on mine.

I don’t think John is referring to combustion air in this case. I think he is referring to a system that requires a certain amount of fresh air to be blended with the conditioned air as part of an air exchange.


That was my take as well. I can’t imagine using flex duct for combustion air. That is if it was connected to a 90+ furnace.

The other end of the duct should terminate in the room with the furnace within 12 inches of the ceiling. I could not find a reason for no screen. The CODE is not too hot on why’s. If the duct extends 6" into the attic and less than 12" through the ceiling it should remain clear at all times. A screen could get blocked with insulation and prevent flow of required air.

true, but with no protection at all, it could get clogged with haycorns, pine cones, wallnuts, chestnets, and it will drive them nuts. dead squrells swell up when stuck in a slick sheet metal tube that there little claws can’t climb. i’d screen it and make it a point to check and clean everytime they replace the fresh air filter. yeah they’ll likely forget both of those, but i’d pick the lesser of 2 evils.

The end of the flex duct terminates into the return side of the system. It connects directly to the return duct. This is not combustion air supply. It’s fresh air being added to the house. My problem with it is the extra work the AC system will have to do to cool all that hot air in the summenr time.
I agree that at the very least a screen needs to be added.

Hi John & Jay!

Here is the information that you were looking for. I ask that you use caution in your inspections.


Only a Code Compliance Inspector can conduct a “Code Compliant Inspection.”

If you start “quoting code” to a client during the physical inspection, and especially in your written report this is an implication that the ENTIRE HVAC system to include duct works, size, type, placement, spacing, support & fastening, vents, the size and placement of the vents, as well as the condenser, compressor, coils, etc. not only meet code but are the correct size to provide sufficient cooling and or heating for the ENTIRE house.


Even the slightest mistake could lead to a lawsuit, and could lead you to “upgrading / replacing” a unit that is too “small” for the home in question.

Once again please be careful about quoting code.

IN ORDER TO PROTECT YOURSELF, Just state that you recommend that a HVAC trained and certified specialist should come and look at the open-ended duct. He / she will see that it should terminate in a “vent”.
[FONT=Times New Roman]This way you have identified a situation that you as a “generalist” feel should be brought to the attention of a “HVAC trained & certified specialist”. [/FONT]

Respectfully, [FONT=Brush Script MT]Frank Carrio[/FONT]
Certified Inspector & Consultant
President, New Hampshire Chapter
The National Association of Certified Home Inspectors {NACHI}

{603} 483-5282

Frank Carrio
P.O. Box 188
Chester, New Hampshire 03036

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806.1 Ventilation required.
Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilating openings shall be provided with corrosion-resistant wire mesh, with 1/8 inch (3.2 mm) minimum to 1/4 inch (6.4 mm) maximum openings.
806.2 Minimum area.
The total net free ventilating area shall not be less than 1 to 150 of the area of the space ventilated except that the total area is permitted to be reduced to 1 to 300, provided at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1 to 300 when a vapor barrier having a transmission rate not exceeding 1 perm (57.4 mg/s · m2Pa) is installed on the warm side of the ceiling.
806.3 Vent clearance.
Where eave or cornice vents are installed, insulation shall not block the free flow of air. A minimum of a 1-inch (25.4 mm) space shall be provided between the insulation and the roof sheathing at the location of the vent.

Hi Frank, good to hear back from you the other day. however, if you look at the pic in the beginning of the post you’ll see that this thread is in concern of a screen on the vent duct to the furnace of this house which either begins or terminates in the attic. i don’t know as if any mention of the attic venting was in question at all. but hay, thanx for the info. it’s alway good to be firmiliar with the code. i personaly never quote code either in my report or communicating with my client. at least if we know the code we can translate that into the ammo we need to call out those deficiencies without actualy mentioning that it is or isn’t code. thanx again Frank, and i’ll be trying to make it up to Concord on the 14th at 10:00.

Must be a northern thing. I’ve never seen make-up air on residential a/c in this area before. I’ve seen plenty on commercial though, usually adjustable from 15% to 40% or so and either manual or automatic and always pulling from the exterior.

If this is connecting to the return side of the system it must be a house make up air duct, it can not by code take the air from the attic it must be from out side unless the local code allows different.

No screens are allowed for make up air or combustion air due to clogging, this does not present an issue that is dangerous for the make up air but it can be very dangerous for combustion air, if blocked the conditions for carbon monoxide then exists.

Here in Ohio make up air is to off set the tighter homes, normally a single 4 inch round pipe is called for, but then its really up to the local code and it can be flex duct. Combustion air is always metal duct by code

An additional note on screens, combustion air pipe size is based on what’s called FREE SPACE or FREE AREA which is the sq inches of unrestricted pipe, a 4 inch round pipe for example has the free space of 13 square inches, when a grill is added it will effectively reduce the free area so it must be taken into account by the designer or a dangerous situation may occur from the reduced free area caused by the grill or screen.

I agree, use caution about code, and always refer to a qualified professional HVAC company

Well guys, I have received lots of responses but no clear answers. I thank you all for your help.
Guess I will call local code inforcement and see what they think.

P.S. Never in my notes here have I quoted or suggested that I was quoting code to the client… How the that part of the thread get started?
Also I was inspecting one of these recently and there was a dead bird right inside next to the filter. How do you suppose he got in there?? Through the open make up air duct unless he got in during original construction.

I know this is what the code says but all of the old diagrams for static combustion air that I have seen requires one high and one low. If you have only one high and the mechanical room/closet is hot and the attic is cold there is going to have to be one huge negative pressure developed in the room to reverse the flow of air that is going into the attic.